The present invention generally relates to endoprosthesis devices, most often referred to as stents, and more particularly pertains to the radiopaque marking of such devices.
Stents or expandable grafts are implanted in a variety of body lumens in an effort to maintain their patency and are especially well-suited for the treatment of atherosclerotic stenoses in blood vessels. These devices are typically implanted by use of a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed which, depending upon its construction, is achieved either automatically by for example the removal of a restraint, or actively by for example the inflation of a balloon about which the stent is carried on the deployment catheter.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First and foremost, the stent must be capable of withstanding the structural loads that are imposed thereon as it supports the lumen walls. In addition to having adequate radial strength or more accurately, hoop strength, the stent should nonetheless be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material of which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent""s structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
Fluoroscopy has typically been relied upon to facilitate the precise placement of a stent as well as to verify the position of a stent within a patient throughout its service life. The use of radiopaque materials in the construction of the stent allows for its direct visualization. Unfortunately, no single material to date has been identified that simultaneously satisfies all requirements inherent in a stent application. Those materials that do satisfy the mechanical requirements are either insufficiently or excessively radiopaque and/or have not been adequately proven to be biocompatible in a vascular setting. Thus, simply constructing a radiopaque stent wholly out of a single material is to date not a viable option. A number of different approaches, however, have been employed wherein different materials are combined in an effort to render a mechanically sound and biocompatible stent to be fluoroscopically visible.
One means frequently described for accomplishing flouroscopic visibility is the physical attachment of radiopaque markers to the stent. Conventional radiopaque markers, however, have a number of limitations. Upon attachment to a stent, such markers may project from the surface of the stent, thereby comprising a departure from the ideal profile of the stent. Depending on their specific location, the marker may either project inwardly to disrupt blood flow or outwardly to traumatize the walls of the blood vessel. Additionally, the galvanic corrosion that might result from the contact of two disparate metals, i.e., the metal used in the construction of the stent and the radiopaque metal of the marker could eventually cause the marker to become separated from the stent which could be problematic should the marker be swept downstream. Finally, although the such markers are typically fairly small, this approach does cause the radiopaque material to come into direct contact with living tissue which may be problematic should there be any biocompatibility issues.
Stents also have been previously marked by plating selected portions thereof with radiopaque material. However, a number of disadvantages are associated with this approach as well. This again causes the radiopaque material to come into direct contact with living tissue which, depending on the total area that is plated, can amount to a sizeable exposure. Additionally, when the stent is expanded and certain portions thereof are caused to undergo substantial deformation, there is a risk that cracks would form in the plating and that sections thereof would become separated from the underlying substrate. This has the potential for creating jagged edges that may inflict physical trauma on the wall tissue or cause turbulence in the blood flowing thereover to thereby induce thrombogenesis. Moreover, once the underlying structural material becomes exposed, interfaces between the two disparate metals become subject to galvanic corrosion. Further, should the plating pattern cover less than all of the stent""s surfaces, the margins between the plated and unplated regions all are subject to galvanic corrosion.
As a further alternative, a stent structure has been described that is formed from a sandwich of structural and radiopaque materials. Three tubes of the materials are codrawn and heat treated to create a structural/radiopaque/structural materials sandwich. Struts and spines are then formed in the tube by cutting an appropriate pattern of voids into the tube as is well known in the art. While this approach does provide a stent that is radiopaque and that fulfills the necessary mechanical requirements, the thin cross section of the radiopaque material is nonetheless exposed along the edges of all cut lines. The biocompatiblity of the radiopaque material therefore remains an issue and more significantly, a sizeable area is thereby created that is subject to galvanic corrosion. Any cuts in the sandwich structure cause two disparate metal interfaces, i.e., thejuncture between the outer structural layer and the central radiopaque layer as well the juncture between the central radiopaque layer and the inner structural layer, to become exposed along the entire lengths of such cuts.
A stent configuration is therefore required that overcomes the shortcomings inherent in previously known devices. More specifically, a stent structure is needed that provides the requisite mechanical properties for such application, that exposes only fully biocompatible materials to living tissue and that is fluoroscopically visible.
The present invention provides a stent that overcomes the shortcomings of previously known stent devices. The stent fulfills all of the mechanical and structural requirements attendant to its function as a stent. Moreover the stent is fluoroscopically visible without any radiopaque material being exposed to living tissue and without any disparate metal interfaces being subject to galvanic corrosion.
The advantages of the present invention are achieved with the complete encapsulation of all radiopaque material that is associated with the stent. In one embodiment a substantially conventional stent is first formed of a structural material by any one of a number of conventional methods. Radiopaque material is then applied to the structure to cover all or just selected portions thereof. A fully biocompatible material is then applied to all surfaces. By fully encapsulating the underlying radiopaque material, any biocompatiblity issues related to the radiopaque material are effectively eliminated as is the potential for galvanic corrosion. Additionally, by ensuring that the outer encapsulating biocompatible skin (or layer) is of sufficient thickness and strength, the risk of compromising the integrity of the skin during the severe deformation that portions of the stent undergo during expansion is also substantially eliminated. Additionally, the outer skin may also be relied upon to contribute to the overall mechanical strength of the stent.
In an alternative embodiment, the underlying stent structure is formed of a radiopaque material that has some or all of the required mechanical properties but may not have been proven to be fully biocompatible in a vascular setting. A structural material that is fully biocompatible is then applied thereto to not only preclude contact between the underlying material and living tissue but to additionally contribute to the mechanical properties of the stent. By fully encapsulating the underlying material, no disparate metal interfaces are exposed to galvanic corrosion.
The radiopaque material may be applied to the underlying structural material by employing any one of a number of well known techniques which include, but are not limited to electroplating, electroless plating, co-drawing and sputter coating. The radiopaque material may be applied to all surfaces of the underlying structure or may be selectively applied so as to form preselected patterns thereon. By advantageously selecting such patterns, the precise orientation or the degree of expansion of the stent may be discernible upon fluoroscopic illumination.
The outer encapsulating skin (or layer) similarly may be applied by any one of the many well known techniques. The material selected for use will usually dictate which method is best suited for its application. Finally, the radiopaque and fully encapsulated stent is subjected to an annealing step wherein an elevated temperature is maintained for a preselected period of time. This serves to enhance the strength of the applied layers and to form strong bonds therebetween.